A common disadvantage of many types of existing power converters with single mode resonant power throughput is the disproportionate voltage and current stress placed on the power train components relative to the level of power throughput. Further disadvantages of existing resonant power converters include limitation to a single switching device (i.e. single ended converters) and limitations on the duty cycle range of the power switch. These single ended converters have a unidirectional flux excursion in the power transformer and hence need the added complexity of a core reset circuit. One such single ended resonant zero current switching power converter having a unidirectional flux excursion in the power transformer is disclosed in U.S. Pat. No. 4,415,959. It therefore requires a commutation switch in order to reset the transformer core and to provide continuous power to the load. It also has the aforementioned disadvantage of limited duty cycle operation in order to prevent internal power circulation and to assure power throughput. This limited duty cycle further causes the peak voltage stress on the power switch, rectifying diodes and resonating capacitor to be excessively large compared to the power being processed.
Power converters such as disclosed by F. C. Lee et al in APEC 86, pages 84-89 in an article entitled "Secondary-side Resonance for High Frequency Power Conversion" are double ended and have a bidirectional resonant path permitting bidirectional flux excursions and hence eliminate a need for a core reset circuit. A particular example described therein employs a resonating capacitor which is shunted directly across the secondary winding. Such an arrangement however still operates with a single mode resonance and places high stress on the power train components. In addition the power circuit disclosed has a tendency to circulate energy in the resonant circuit thereby blocking energy flow to the output if the duty cycle of the power switch is too large (i.e., greater than 50%) or if operated at very light load.
It is apparent that the above described prior art exemplary converters operate in single mode resonance and place a high stress on power train components. Attainable power levels for given component capacities is therefore limited. A tendency to circulate energy within a converter tank circuit at certain duty cycles is a further limit on the versatility of these prior art converters.